September 2017 Update

Task/Combinations/JavaScript/combinations-6.js

@@ -1,47 +1,65 @@
-(function (n) {
-    'use strict';
+(() => {
 
-
-    // n -> [a] -> [[a]]
-    let comb = (n, xs) => {
-            if (n < 1) return [[]];
+    // combinations :: Int -> [a] -> [[a]]
+    const combinations = (n, xs) => {
+        const cmb_ = (n, xs) => {
+            if (n < 1) return [
+                []
+            ];
             if (xs.length === 0) return [];
-
-            let h = xs[0],
+            const h = xs[0],
                 tail = xs.slice(1);
+            return cmb_(n - 1, tail)
+                .map(cons(h))
+                .concat(cmb_(n, tail));
+        };
+        return memoized(cmb_)(n, xs);
+    }
 
-            return comb(n - 1, tail)
-                .map((t) => [h].concat(t))
-                .concat(comb(n, tail));
-        },
+    // GENERIC FUNCTIONS ------------------------------------------------------
 
+    // 2 or more arguments
+    // curry :: Function -> Function
+    const curry = (f, ...args) => {
+        const go = xs => xs.length >= f.length ? (f.apply(null, xs)) :
+            function () {
+                return go(xs.concat(Array.from(arguments)));
+            };
+        return go([].slice.call(args, 1));
+    };
 
+    // cons :: a -> [a] -> [a]
+    const cons = curry((x, xs) => [x].concat(xs));
 
-        // Derive a memoized version of a function
-        // Function -> Function
-        memoized = (f) => {
-            let m = {};
+    // enumFromTo :: Int -> Int -> [Int]
+    const enumFromTo = (m, n) =>
+        Array.from({
+            length: Math.floor(n - m) + 1
+        }, (_, i) => m + i);
 
-            return function (x) {
-                let args = [].slice.call(arguments),
-                    strKey = args.join('-'),
-                    v = m[strKey];
+    // Derive a memoized version of a function
+    // memoized :: Function -> Function
+    const memoized = f => {
+        let m = {};
+        return function (x) {
+            let args = [].slice.call(arguments),
+                strKey = args.join('-'),
+                v = m[strKey];
+            return (
+                (v === undefined) &&
+                (m[strKey] = v = f.apply(null, args)),
+                v
+            );
+        }
+    };
 
-                return (
-                    (v === undefined) &&
-                    (m[strKey] = v = f.apply(null, args)),
-                    v
-                );
-            }
-        },
+    // show :: a -> String
+    const show = (...x) =>
+        JSON.stringify.apply(
+            null, x.length > 1 ? [x[0], null, x[1]] : x
+        );
 
-        range = (m, n) =>
-            Array.from({
-                length: (n - m) + 1
-            }, (_, i) => m + i);
-
-
-    return memoized(comb)(n, range(0, 4))
-
-
-})(3);
+    return show(
+        memoized(combinations)(3, enumFromTo(0, 4))
+    );
+})();